In his early works, such as the museum at The Arsenal in Vienna, Hansen was still rather aligned to a more romantic style. In later years, he became the most outstanding representative of Renaissance-inspired historicism (Neo-Renaissance), which also came to be known as Viennese-style. This style extended into the smallest details of the interior design and partially accepted the courses of a synthesis of the arts.

Along with Förster and many others, Hansen was one of the most important and influential architects of the Viennese Ringstraße. His most famous work is the Austrian Parliament building, which was created in the style of an ancient, neo-classic temple, and serves to refer to the Greek beginnings of democracy. Hansen was originally a staunch critic of the Classical style that was taught to him at the Copenhagen Academy. Over the years, however, he came to incorporate Classical elements into his forms. Bauleiter on this project was Hans Auer, who would go on to win the competition for the Swiss Bundeshaus.

Hansen's famed Musikverein in Vienna is one of the most notable concert halls in the world; a concert hall whose design and acoustics are often admired and copied in present-day music houses.